The invention relates to a process for producing valve metal powders, in particular tantalum powders, by reduction of a suitable valve metal compound with an alkali metal, the reduction being carried out in the presence of a grain-refining agent which is added in portions or continuously.
Valve metals, which are to be understood as meaning in particular tantalum and its alloys, as well as other metals from Group IVb (Ti, Zr, Hf), Vb (V, Nb, Ta) and VIb (Cr, Mo, W) of the periodic system of the elements and alloys thereof, are in widespread use in component manufacture. The use of niobium or tantalum for the production of capacitors should be particularly emphasised.
The starting point for the production of niobium or tantalum capacitors is usually corresponding metal powders, which are initially pressed and then sintered in order to obtain a porous body. The latter is anodised in a suitable electrolyte, so that a dielectric oxide film is formed on the sintered body. The physical and chemical properties of the metal powders used have a crucial influence on the properties of the capacitor. Crucial characteristics include, for example, the specific surface area and the level of impurities.
Tantalum powder of a quality which allows it to be used to produce capacitors is usually produced by sodium reduction of K2TaF7. In this process, K2TaF7 is placed in a retort and is reduced by liquid sodium. In this reaction, it is particularly important to control the grain size and therefore the specific surface area, since these properties determine the specific capacitance of the capacitors produced therefrom. The finer the grain, the higher the specific charge.
It is known from U.S. Pat. No. 5,442,978 that the grain fineness can be influenced by the following factors:    1. A high reaction temperature results in more rapid grain growth and therefore in the formation of a coarser grain.    2. An excess of reducing agent during the reaction leads to the formation of a large number of crystallisation nuclei. Rapid addition of sodium is therefore advantageous.    3. A high dilution of K2TaF7 in a molten salt leads to the formation of a large number of individual crystallisation nuclei and is therefore advantageous.
Therefore, to produce tantalum powder with a high specific surface area, U.S. Pat. No. 5,442,978 proposes the production of highly dilute K2TaF7 by the stepwise addition of sodium, the addition being carried out at a high rate. There are no details as to the specific surface areas which can be obtained using this process. During this process, non-uniform concentration ratios of the reactants occur during the reaction. The K2TaF7 concentration is initially high but drops constantly as the sodium is added, so that the grain size distribution of the powder which is formed is very wide.
According to U.S. Pat. No. 4,684,399 it is advantageous for the tantalum compound to be added continuously or in steps during the reduction. As a result of this measure, the concentration of tantalum compound remains more uniform during the reduction process. It is also preferable for the reducing agent sodium to be added continuously or in steps.
DE 33 30 455 A1 also describes a process for producing valve metal powders with the objective of obtaining powders with a fine grain size and a large surface area. For this purpose, a reaction mixture comprising reduction metal and double fluorine salt of the valve metal is reacted in the presence of a doping element. The proposed doping element is elemental sulfur or a sulfur compound, for example Na2SO4. The reaction partners are introduced together and reacted in a batch reaction. The valve metal powders obtained have BET surface areas of up to 0.64 m2/g.